The present invention relates to machines for weighing and classifying tablets, particularly at a high rate of speed. The invention utilizes the basic principle of capacitance, wherein the weight of a tablet can be determined within precise limits by passing the tablet through a capacitance sensor, and measuring the change in capacitance which results from the presence of the tablet in the sensor.
In the prior art, high-speed machines have been developed for classifying pharmaceutical capsules, as in U.S. Pat. No. 4,223,751, issued Sep. 23, 1980, and owned by the assignee of the present invention. A further and related patent is found in U.S. Pat. No. 4,402,412, issued Sep. 6, 1983. In both of the foregoing patents the weight of a capsule in a stream of capsules is determined by passing the moving capsule through a fixed capacitive sensor arranged in tubular form, wherein the resultant change in capacitance provides a measure of capsule weight. A deflector mechanism is provided downstream in the path of capsule travel, and the capsules which are measured outside of certain weight parameters are deflected from the path of travel into a reject bin. Capsules which measure within the desired weight range are permitted to continue in the path of travel to be received in a second bin.
The basic capacitance principles of the foregoing patents are utilized in the present invention albeit in a different constructional form, in order to accommodate the different problems which arise in connection with measuring the weight of tablets. Whereas a capsule is constructed of uniform form, by placing a measured quantity of powder inside a preformed housing, a tablet tends to be of nonuniform shape and of varying size. Capsules are symmetrical about a longitudinal axis and are therefore amenable to propagation through a tubular passage, whereas tablets are of somewhat nonuniform disk shapes, having a predetermined diameter and varying thickness dimensions. When tablets are propagated through a tube they tend to tumble and jam up in the flow path, thereby rendering devices of the type disclosed in the prior art patents useless for weighing and classifying tablets.
In attempting to apply the machines of the type disclosed in the foregoing patents to the problem of weighing tablets, it became apparent that an entirely different approach to the design of the transport and sensing mechanism would have to be utilized, and the present invention resulted from the study of applying capacitance measuring techniques to a high-speed moving stream of tablets.